1. Field of the Invention
The present invention relates to a system for washing and film-forming a semiconductor wafer continuously.
2. Description of the Related Art
When the semiconductor wafer reacts with oxygen in the air in the semiconductor devices producing process, native oxide is created on the surface of the wafer. This native oxide increases the thickness of gate oxide film and the contact resistance. It is therefore needed that the native oxide is removed from the wafer to stop its growth before the following film-forming process is applied to the wafer.
When the wafer is loaded into the process tube in a case where the film-forming process is applied to it under high temperature, air is swallowed up into the process tube by convection. As the result, the wafer reacts with oxygen to develop the growth of the native oxide. It has been therefore discussed these days that the wafers are loaded into and unloaded from the wafer boat while filling the lower area of the vertical process tube with the atmosphere of inactive gas.
On the other hand, semiconductor devices are more and more highly intergated and the capacity of a DRAM is about to increase from 4M to 16M, 32M and more. In order to meet this state, the thickness of oxide film must be controlled with higher accuracy and excessive native oxide must be removed when capacitor film is to be formed.
Accordingly it is not sufficient that the growth of the native oxide is stopped as seen in conventional cases only when the wafer is loaded into the process tube. But it is needed to remove the native oxide of about 10 .ANG. which is caused when silicon reacts with oxygen and water in the air while the wafer is being carried from the washing section to the film-forming station.
Even when the native oxide is once removed from the wafer, it grows again while the wafer is on its way to the following station because the film-forming station is located remote from the washing station in the conventional system. Further, the wafer which has been washed are often kept waiting because the timing at which the wafer is processed in the washing station is quite different from those timings at which it is processed in the other stations.
The native oxide which had re grown in this manner was conventionally negligible because it was quite thinner than such native oxide that was caused when the wafer was loaded into the process tube. When a DRAM comes how to have a capacity larger then 32M, however, the thickness of oxide film must be controlled with a higher accuracy and excessive native oxide must be removed from the wafer even when capacitor film is to be formed. Even the growth of quite thin native oxide must be stopped under these circumstances.